Many industrial processes use sulfuric acid to promote reaction. In such use they generate contaminated dilute streams of sulfuric acid which must be reconcentrated, cleaned-up, neutralized or otherwise disposed of. Typical of such processes are organic nitrations where the affinity of sulfuric acid for water allows nitro groups to be placed on both aliphatic and aromatic organic compound. Explosives, dyestuffs, and many fungicide, herbicide and insecticide processes use such techniques. Dilute sulfuric acid is also produced where the acid is used to dry a gas stream such as chlorine or sulfur dioxide.
The dilute sulfuric acid streams which must be disposed of may contain impurities, some of which are reaction by-products. One technique for removal of the impurities is to raise the temperature of the acid to between 250.degree. and 330.degree. C. and to hold the acid at this temperature until the impurity has been digested by the hot acid. Very hot sulfuric acid is a powerful oxidant, provided that its temperature is high enough.
Various approaches have been used in the past to reconcentrate dilute sulfuric acid, but the costs have been high and problems have existed. One common approach is the use of pot concentrators, where contaminated acid is passed through a scrubbing column into a cast iron pot which is heated by products of combustion to evaporate vapour which then passes out through the scrubbing column to atmosphere. This approach operates at atmospheric pressure. The process offers very long residence time of the hot acid in the pot but has a very limited ability to evaporate water since the amount of heat that can be transferred to the acid is limited by the exposed surface for heat transfer, the very limited external heat transfer coefficient associated with atmospheric pressure hot gas, and the thermal resistance of the wall of the cast iron pot. A further disadvantage is that failures of the cast iron pot have not uncommonly led to large quantities of very hot acid being dumped into the furnace and environment, causing extensive and visible pollution.
A second approach which has been used in the past is to blow combustion gases at atmospheric pressure into the waste acid. While this approach is effective in injecting heat into the acid, it has been found to generate an off-gas containing a very persistent and noxious chemical fume which is difficult to remove. This approach has therefore fallen into disfavour.
A third and more common approach is the use of heat transfer surfaces in tantalum or glass (e.g. glass-lined vessels). In this case high pressure steam is commonly used as the energy source. However with the use of tantalum, there are severe limits on the temperatures that can be used in the evaporation. These limits are set by the corrosion of tantalum which must be kept low to avoid the nascent hydrogen corrosion product from accumulating in the tantalum and causing hydrogen embrittlement and equipment failure. Typical maximum acid temperatures of 190.degree. C. therefore exist when tantalum is used. Glass lined steel usually has a safe upper temperature operating limit of about 230.degree. C.
When the 190.degree. C. temperature is combined with vapour pressure tables for sulfuric acid solutions, it is found that the vapour pressures that can be accepted over acid above 90% strength are in the order of 10 mmHg. Such pressures cannot be obtained in a barometric condenser, so it has been found necessary in such cases to use steam jet ejectors to raise the pressures to the level at which the vapors can be condensed against cooling water. Steam jet ejectors are normally not preferred because such ejectors can use between one and five pounds of high pressure steam for every pound of vapour condensed. The density of water vapour at pressures of 10 mmHg is also very low, and even using the largest glass lined equipment, the capacity of a single vessel as an evaporator is very limited, so frequently multiple units are required.
A further limitation of the above approach is that there is a significant content of sulfuric acid in the vapour over acid of about 80% strength, and simple removal of vapour can then transfer such acid into effluent and condensate systems where it is undesirable. In some designs this problem and the very low vapour pressure have been accommodated by condensing the vapors from the last and strongest effect in cold feed acid which has a much lower vapour pressure than water. This last approach eliminates the need for steam jet ejectors and the worry about sulfuric acid vapors contaminating the condensates. However it brings a new inefficiency to the process since the water vapour from the last effect now needs to be evaporated twice.
A still further limitation to the above process lies in the frequent need to raise the acid to temperatures above 250.degree. C. to destroy organic by-products present in the acid. Such temperatures are impossible to achieve with tantalum heat transfer surfaces.
Other vacuum approaches exist using acid resistant brick, lead and glass equipment but suffer similar constraints to those of the cast iron pot with respect to capacity, and they are expensive because of the need to use large vessels resistant to hot concentrated acid.